It’s lonely out here in the galactic suburbs. The closest neighbors are light-years away — distances that are truly astounding. The distance to the Sun’s nearest stellar neighbor, Proxima Centauri, is almost 29 million times the Sun’s diameter. At that same scale, if you live in a house that’s 50 feet wide, your nearest neighbor would be more than a quarter-of-a-million miles away — farther than the Moon. That’s a pretty typical gap between star systems in this part of the galaxy — although not necessarily between individual stars. While the Sun is alone, many other stars have close companions. Proxima Centauri, for example, belongs to a system of three stars. The space between the stars isn’t completely empty. It contains wisps of hydrogen and helium gas, plus a tiny smattering of heavier elements. On average, there’s about one atom of gas for every cubic centimeter of space. There’s a lot more stuff packed together in the giant clouds known as nebulae. If they have enough gas and dus
